Effect of Aquifer Size On the Performance of Partial Waterdrive Gas Reservoirs

Predicting the advancement of a gas/water contact (GWC) in a waterdrive gas reservoir plays an important role in evaluating, forecasting, and analyzing the reservoir performance. This study was conducted to predict the behavior and the rise of the GWC, assuming that it remains horizontal, and to determine its effect on ultimate gasrecovery. Several factors control the rise of the GWC. Some of the most important factors are the size of the aquifer, gas production rate, initialreservoir pressure, and formation permeability. These factors account for the abandonment of a number of gas reservoirs at extraordinarilyhigh pressure. Several methods have been developed for predicting the volume of water influx into a reservoir; the van Everdingen-Hurst method is used inthis study. The performance calculated in this study was based on the material-balance equation for gas reservoirs. The gas reservoir pressurewas adjusted to the original GWC for the water-influx equation, and the trapped gas in the water-invaded zone was accounted for in the waterinvadedregion. A constant reservoir permeability of ٣٠٠ md was used in all calculations. The results showed that when ra/rg≤٢, the effect of theaquifer on gas reservoir performance can be neglected. Also, the rate at which the GWC advances is controlled by the aquifer size when ra/rg >٢.Finally, regardless of the size of the reservoir, when ra/rg >٢, the pressure in the unsteady-state water-influx equation has to be corrected to the original GWC. Failure to do so may result in an error of more than 3rd Iranian Petroleum Engineering Congress ١٠٠% in the cumulative water influx, which in turn could lead to the wrong conclusions regarding the performance of the gas reservoir.

کلیدواژه ها:

gas ، water contact (GWC) - size of the aquifer ، gas production rate - initial reservoir pressure - formation permeability - van Everdingen&Hurst method